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Author Topic: Soundcard or TNC  (Read 2429 times)
N9PCS
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Posts: 32




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« on: September 15, 2004, 12:44:08 PM »

I am returning to amateur radio after a 10 year absence. Even though I had upgraded to advanced in 1993, I was never active on anything other than 2m FM and 2m packet. I had always hoped to try RTTY and some of the HF digital modes, but never completed the station. I did own a PK-232 TNC (now lost after several moves).

I am now at a QTH that will allow an HF antenna and I finally own a HF transceiver. I would like to finally get around to getting on HF. I have been reading some about PSK31, and it seems almost all the digital modes (including packet) can now be done with a soundcard instead of a TNC. Yet I see an upgraded DSP version of the 232 TNC selling for over $500 new. I don't quite get it.

I guess I am looking for the pros and cons of soundcard vs TNC for the digital modes.



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W4TME
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Posts: 299




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« Reply #1 on: September 15, 2004, 12:52:04 PM »

Can't do PSK with a TNC unless it has special capabilities to do such.  You can do a lot more for a lot less (even packet & APRS which is what you would use the TNC for) with a $20 soundblaster card and MixW.

The TNC will decode packet a little better than the soundcard at marginal signal levels.

-Tim W4TME
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N8UZE
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Posts: 1524




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« Reply #2 on: September 15, 2004, 04:27:07 PM »

Each has its advantages and disadvantages.  Going the sound card route is easy and inexpensive.  However with the separate TNC, the decoding of the signal is in the TNC itself and not the computer.  Then the TNC sends ASCII data back to the computer.  This puts very little load on the computer, making it easier to run multiple applications while running your digital modes.  With the sound card/software approach, all the processing is done by the computer and thus reduces the number of applications you can have running simultaneously.  In addition, the decoding portion of the software has to have priority for the real time conversions and this can slow down any other applications you have running at the same time.  This may or may not be significant for you.
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AA4PB
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Posts: 15022




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« Reply #3 on: September 15, 2004, 10:12:09 PM »

There are a number of reasons one might want to have a stand alone TNC or hardware controller. If you want to run Pactor or Amtor ARQ then you have to have a hardware controller because the required timing is too critical for Windows. A hardware TNC also lets you run it "stand alone" for unattended operation (mailbox, etc) without the need to have a computer running.
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Bob  AA4PB
Garrisonville, VA
N1ZZZ
Member

Posts: 213




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« Reply #4 on: September 15, 2004, 11:45:36 PM »

Besides being much less expensive, there are far more people on the soundcard modes then the ARQ modes.  If your thing is keyboard to keyboard QSO's the soundcard is the way to go.  If you want to do a lot of data links (BBS and Email) you might look into a TNC.

73
Jeremy N1ZZZ
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N4ZOU
Member

Posts: 340




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« Reply #5 on: September 20, 2004, 01:32:16 PM »

One thing everyone forgets about a TNC is the built in filters for the mark and space tones. Some TNC's have good filtering that is switched in and out for different modes like HAL equipment. Even the poor filtering in a PK-232 TNC, which is set up for Packet mode is still better than the wide-open front end of a sound card. Serious RTTY contesters use hardware equipment for RTTY mode for that reason. Terminal Units from HAL are a good example and will allow copy of text in conditions where you can't even hear the signal and have stations almost on top of that station in contest conditions. Do not get me wrong; I also use a sound card for digital modes that can only be used for that mode like MFSK and PSK-31. With my setup I have a HAL ST-6000 plugged into the external modem port on a PK-232MBX and I also have all the required interface circuits in a box so that I can switch between the PK-232MBX internal modem for Packet, 200 Baud Pactor, and other modes with a wider bandwidth or the ST-6000 with it's very narrow active filtering system. I can also switch to the sound card in the computer. It's the best of all digital worlds. So do not limit yourself to just a sound card or a TNC, do both! Start out with a sound card and when you run up on a good deal for a TNC simply add it to your digital setup. Then if you run up on a high performance HF modem you can add it to a PK-232 external modem port so you can use it with your modern computer system. I use a computer with only USB ports and a cheap USB to serial adaptor works just fine with my PK-232MBX TNC. The system works great with N1MM contest software as well. Here is a link to an article detailing this. http://www.eham.net/articles/8616

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AA4PB
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Posts: 15022




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« Reply #6 on: September 22, 2004, 01:01:47 PM »

So does a sound card have built in filters. The DSP filters used with the sound card are probably better than most of the L/C or switched capacitor filters used in all but the most expensive TNCs.

Both the TNC (or more correctly multimode controller) and the sound card work at the audio level so neither can do anything about the wide open IF in the receiver. The most effective filter is an IF filter but that is not related to the choice of sound card or TNC.
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Bob  AA4PB
Garrisonville, VA
N4ZOU
Member

Posts: 340




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« Reply #7 on: September 25, 2004, 04:52:55 PM »

By the time the audio gets to the DSP filters in a sound card it's too late. I have checked this by using external audio filters on the input of a sound blaster 16 sound card. Try it yourself. Even an old active audio filter like an Autek QF-1 or MFJ-784B made for connection to a speaker makes a big improvement in performance of the sound card. I have never understood why some of the sound card interface makers to not put even a simple OP-AMP filter in the audio line between the transceiver audio output and the sound card line-in jack except then you would only see a very narrow part of the water fall tuning display when it's switched in. That alone will show you the audio bandwidth that is open to the front end of the sound card. Any signal present in the tuning display that is very close or overdriven will cause distortion of every other signal in that display. You must choke that off before it gets to the sound card audio input. A high performance terminal unit has very narrow active filter circuits for both the mark and space tones! Not just a single filter so that both the mark and space tones are allowed though like a transceiver but two very narrow filters for just the mark and just the space tones. This will even take care of an interfering tone between the mark and space tones. The only thing that comes close is the twin peak RTTY filters in the latest DSP filter model Icom transceivers. Then there are the active circuits so you can turn the AGC control completely off in the transceiver and let the high performance terminal unit take care of the audio levels where it counts so no close distorted station will wipe out the tones your trying to copy.    
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